Fuel cell systems are configured to: supply a hydrogen-containing gas and an oxygen-containing gas to a fuel cell stack (hereinafter, simply referred to as a “fuel cell”) which is the main body of the system's power generating part; cause an electrochemical reaction between hydrogen and oxygen to progress; and extract chemical energy generated through the electrochemical reaction as electrical energy to generate electric power. Fuel cell systems are capable of generating electric power with high efficiency, and allowing thermal energy generated during a power generation operation to be readily utilized. Therefore, fuel cell systems are being developed as distributed power generation systems that make it possible to realize highly efficient energy utilization.
Generally speaking, it is often the case that an infrastructure serving as the source of supply of the hydrogen-containing gas is not developed. Therefore, conventional fuel cell systems are provided with a hydrogen generation apparatus. Such a hydrogen generation apparatus includes a reformer configured to generate a reformed gas (a hydrogen-containing gas) (see Patent Literature 1, for example). The hydrogen generation apparatus uses water and a raw material. The raw material is, for example, city gas which contains natural gas as a main component and which is supplied from an existing infrastructure, or LPG. The hydrogen generation apparatus causes a reforming reaction between the water and the raw material at temperatures of 600° C. to 700° C. by using a Ru catalyst or a Ni catalyst, thereby generating the reformed gas. Usually, the reformed gas obtained through the reforming reaction contains carbon monoxide, which is derived from the raw material. If the carbon monoxide concentration in the reformed gas is high, it causes degradation of the power generation performance of the fuel cell. Therefore, it is often the case that, in addition to the reformer, the hydrogen generation apparatus includes reactors, for example, a shift converter and selective removers such as a selective oxidizer and a methanation remover. The shift converter includes a Cu—Zn based catalyst, and causes a shift reaction between carbon monoxide and steam to progress at temperatures of 200° C. to 350° C., thereby reducing carbon monoxide. The selective oxidizer selectively causes a carbon monoxide oxidation reaction at temperatures of 100° C. to 200° C., thereby further reducing carbon monoxide. The methanation remover selectively causes carbon monoxide methanation, thereby reducing carbon monoxide.
In the hydrogen generation apparatus, there are cases where aging variation of a reforming water pump and a raw material flowmeter, which are included in the source of supply of the reforming water and the raw material, causes a temporal change in the ratio between the supplied raw material and the supplied water. In this case, a change in steam/carbon ratio occurs. If a significant abnormality occurs in the steam/carbon ratio, it becomes difficult to cause a proper reaction in the reformer. This may consequently cause, for example, problems as follows: the hydrogen-containing gas content in a fuel gas supplied to the fuel cell decreases, or the carbon monoxide gas content in the reformed gas after passing through the shift converter increases. Moreover, in some cases, the power generation voltage of the fuel cell drops, which may cause the fuel cell to stop. Therefore, there is proposed a method for determining an abnormality that the steam/carbon ratio is high (see Patent Literature 2, for example).